1. Field of the Invention
The subject invention relates to polymerizable mixtures suitable for the production of polymers having enhanced thermal stability, methods for producing such polymers, and the resulting polymers. The invention also relates to a novel sucrose derivative useful in the polymerizable mixtures.
2. Description of the Related Art
The need to stabilize polymers to increase their usefulness, for example, for high-temperature applications, has long been known in the art. One unsuccessful method of stabilizing polymers tried was the use of crosslinking agents in the synthesis of polymers. See, Grassie et al., Proc. Royal. Soc. Lond., A199, 14 (1949). Compounds such as glycerol, sorbitol, 3,5-dihydroxy-methylbenzyl alcohol and pentaerythritol find wide applicability as polyfunctional crosslinking agents.
A crossing agent which is commonly used is 2-ethyl-2-hydroxymethyl-1,3-propanediol or trimethylolpropanetriol. This crosslinking agent, or trifunctional crosslinking agent as it may more accurately be described, is used in particular to produce crosslinked polyesters.
It has additionally been known to functionalize trimethylolpropanetriol further to produce other trifunctional crosslinking agents. For example, it is known that treatment of 2-ethyl-2-hydroxymethyl-1,3-propanediol with acryloyl or methacryloyl chloride will produce 2-ethyl-2-hydroxymethyl-1,3-propanediol triacrylate and 2-ethyl-2-hydroxymethyl-1,3-propanediol trimethacrylate. The resulting polyfunctional crosslinking agents find particular applicability in crossing reactions with acrylates and methacrylates.
In such methods, the trimethylolpropane triacrylate or TMPTA and trimethylolpropane trimethacrylate or TMPTMA are then reacted with acrylate and methacrylate monomers, preferably in the presence of a sufficient amount of free radical initiator to facilitate polymerization. Such methods are often used to produce crosslinked thermoset polymers. The resultant thermoset polymers find applicability, e.g., in making automobile coatings and composites. Additionally, crosslinked thermoset polymers are used in bulk, or monomers may be diluted with other compounds to produce adhesives or composite materials.
Despite their widespread use and commercial importance, trimethylolpropane triacrylate or trimethacrylate, pentaerythritol triacrylate or trimethacrylate, ethyleneglycol bisacrylate or bismethacrylate, and methylene bisacrylamide are exceedingly inefficient at crosslinking because of their proclivity to form rings by intramolecular conjugate addition of radicals. Such low crosslinking efficiencies may result in polymers which are not structurally load bearing. Matsumoto and colleagues have shown that trimethylolpropane trimethacrylate (TMPTMA) undergoes intramolecular loop formation 82% of the time, implying an 18% efficiency of crosslinking (Matsumoto, A. et al, European Polymer Journal, 1989, Vol. 25, pp. 385-389). Intramolecular loop formation by conjugate addition of radicals should be expected in all crosslinking agents (acrylates, methacrylates, and acrylamides) wherein two mutually reactive functional groups are spanned by 1 to 4 atom bridges. This is because, in such systems, there is no conformational or electronic bias precluding the formation of medium-sized bislactone or bislactam rings. Neither is the formation of macrocycles containing a crosslinker and several monomers precluded by radical mechanisms, as shown below. ##STR1##
Furthermore, when acrylic or methacrylic copolymers containing commercial crosslinkers are heated at or beyond their ceiling temperatures (ca. 200.degree. C.), they degrade rapidly. This is because commercial crosslinkers contain no structural elements or mechanisms that quench radicals and stabilize a depolymerizing network.
In order to minimize the recognized problems associated with such crosslinking agents, it had been contemplated in the art to increase the overall concentration of the crosslinking agent in the polymerization mixture as a means of enhancing the degree of crosslinking. However, at high crosslinking agent concentrations, the resultant polymers become brittle and mechanically Weak. Additionally, the increased concentration of the crosslinking agent renders the polymers undesirably expensive to produce. For instance, it is described, e.g., in Matsumoto et al, "Gelation in the copolymerization of methyl methacrylate with trimethylolpropane trimethacrylate.", Eur. Polym. J., Vol. 25 (4), pp. 385-389 (1989), that the crosslinking efficiency of trimethylolpropane trimethacrylate is only 18% and that 82% of the crosslinker had internally cyclized despite the fact that an excess of methyl methacrylate was present in the polymerization mixture.
Furthermore, K. Dusek in Developments in Polymerization-3, R. N. Howard (editor), Applied Science Publishers, Ltd., London, p. 63, (1982), describes the crosslinking efficiency of methylene bis-acrylamide to be about 10-20% and about 10-20% by weight of methylene bis-acrylamide is needed to effect network formation in hydrogels.
Besides the use of crosslinking agents, other methods of stabilizing polymers have also been utilized. For example, two general stabilizing mechanisms exist for methyl methacrylate polymers by which thermal depropagation may be temporarily arrested. These include (a) blocking depropagation by use of a comonomer such as methyl acrylate, or (b) disrupting depropagation of the PMMA macro radical by use of a radical quencher (see, I. C. McNeill, Comprehensive Polymer Science, I. C. McNeill, Ed., Vol. 6, Pergamon Press, New York, pp 451-500, 1989; N. Grassie, Chemistry of High Polymer Degradation Processes, Interscience Publishers Inc., New York, 1956; N. Grassie et at, Chem. Zvesti, 26, 208 (1972); N. Grassi and G. Scott, Polymer Degradation & Stabilisation, Cambridge University Press, London, pp 1-67, 1985; D. H. Solomon, J. Macromol. Sci.-Chem., A17, 337 (1982); N. Grassie, Pure & Appl. Chem., 30, 119 (1972); N. Grassie and H. W. Melville, Proc. Royal. Soc. Lond., A199, 14 (1949); K. A. Holland and I. D. Rae, Aust. J. Chem., 40, 687 (1987); A. Meisters et at, Poly. Bull., 20, 499 (1988); and T. Kashiwagi et al, Combustion and Flame, 81, 188 (1990)). Onset of degradation in PMMA copolymers may be delayed up to 300.degree. C. by three methods: (a) copolymerization of MMA with monomers that do not depropagate easily (see, N. Grassie et at, Eur. Polym. J., 17, 589 (1981); N. Grassie et al, Polym. Degradation Stab., 16, 19 (1986); J. Popovie et at, Polym. Degradation Stab., 32, 265 (1991); K. Hayakawa et at, J. Appl. Polym. Sci., 29, 4061 (1984); I. B. Shafranskaya and G. P. Gladyshey, Izv. Akad. Nauk. Kaz. SSR, Ser. Khim., 8, 73 (1968); Y. I. Puzin et al, Vysokomol. soedin., Ser. B, 29, 183 (1987); J. San Roman et al, Rev. Plast. Mod., 31,213 (1976); and V. S. Nikiforenko et at, Plast. Massy, 59 (1988)); (b) polymerization of MMA in the presence of chalk fillers (see, K. F. Paus et al, Plast. Massy, 13 (1986)), or in porous glass matrices (see, F. M. Aliev and V. N. Zgonik, Eur. Polym. J., 27, 969 (1991)); and (c) polymerization of MMA in the presence of disulfide additives (see U.S. Pat. No. 3,978,022). Thermal stability beyond 300.degree. C. may be provided to PMMA by: (a) anionic polymerization (see, T. Kashiwagi et al, Macromolecules, 19, 2160 (1986)); (b) treatment of PMMA with hydrogen in the presence of a catalyst (see, N. Grassie and H. W. Melville, Proc. R. Soc. London, A199, 14 (1949); H. H. G. Jellinek, Degradation of Vinyl Polymers, Academic Press, New York, 74, 1965; I. C. McNeil, Eur. Polym. J., 4, 21 (1968); and P. Cacioii et al, Polym. Bull. (Berlin), 11, 325 (1984)) or ozone at -78.degree. C. (see, Y. Murashige, Japanese Patent 03045604A2, Feb. 27, 1991, Chem. Abst. 115(2): 9698j), to destroy terminal olefins; (c) copolymerization with 25 to 50 mole percent styrene (see, A. Kaminska et al, Polym. Networks Blends, 1, 165 (1991)); and (d) blending PMMA with 25 to 50 weight bisphenol-A polycarbonate (see, A. Rincon, I. C. McNeill, Polym. Degradation and Stab., 18, 99 (1987); and S. H. Goh, Thermochimica Acta, 153, 423 (1989)). None of these methods has provided an efficient, simple method for the thermal stabilization of polymers. For example, none of these methods is known to provide long-term thermal stability and many of the methods require a large mount of additive to stabilize PMMA. See, for example, Goh and Rincon et al. Moreover, these methods of stabilization may be quite expensive in practice. Thus, there is an urgent need for thermal stabilizers that will extend the life of the polymer when subjected to heat.
The importance of polymeric stabilizers may be seen in the use of present day petrochemically-derived acrylic and methacrylic crosslinkers in the manufacture of windows, skylights, windshields, insulators, paints, coatings, golf ball cores, and bathroom utility items such as sinks, bathtubs and panels (CORIAN.RTM.). The acrylamide crosslinker, for example methylene bisacrylamide, is used in the manufacture of water purification gels, ion-exchange resins, electrophoresis gels, "super slurper" gels, and corrosion inhibitors. The allyl crosslinker is used as a second stage crosslinking agent of vinyl polymers. These vinyl polymers in turn are used for making water-based paints; adhesives for paper, textiles and wood; coating compounds; and thickening agents.
Thus, given the described state of the prior art and the various uses for stabilized polymers, it is clear that improved methods for producing stabilized polymers and improved stabilized polymers resulting from these methods would be highly desirable. In addition to improved methods for producing thermally stable crosslinked polymers, improved non-crosslinked polymers resulting from these methods would be highly desirable.